The present invention relates generally to lens systems, and more specifically to techniques for correcting aberrations such as field curvature.
The presence of a considerable amount of field (Petzval) curvature is a relatively basic feature of most refractive lens systems. However there are certain applications where a flat field is important, including camera objectives and some relay optics. Unfortunately, field curvature is one of the hardest aberrations to correct, requiring increases in the complexity of the lens far in excess of those required to correct spherical and chromatic aberrations. Other aberrations such as astigmatism are also a function of the field position and may be difficult to correct in the presence of other constraints to the lens design.
The present invention provides a powerful and cost-effective technique for correcting optical systems for a variety of aberrations, or for otherwise controlling the imaging properties of the optical system.
In short, the invention uses a microlens array where an optical property of the individual microlenses varies as a function of position in the array. A primary example is where the microlenses are configured so that the focal length varies to correct field curvature. More specifically, in a two-dimensional array, the focal lengths of the microlenses vary as a function of distance from the center of the array. Typically, the focal length would be smallest near the center and increase with distance from the center, but the converse could be true if it was desired to increase the field curvature for some special purpose.
Representative environments for a microlens array according to the invention may be characterized by a pixellated source (array of individual sources) in an object plane or a pixellated detector (array of individual detectors) in an image plane, or both. In this context, the individual sources or detectors could be the ends of optical fibers.
The invention can also be used to correct other aberrations such as astigmatism by configuring the spatial variation appropriately. While the focal length is one microlens property that can be spatially varied, the shape can also be varied, for example by a departure from a spherical surface contour.
A further understanding of the nature and advantages of the present invention may be realized by reference to the remaining portions of the specification and the drawings.